In wireless communications systems with multiple antennas, e.g., OFDM-based cellular systems with multiple antennas, it is desirable to send multiple layers of data over the same frequency-time resource to fully utilize the spatial dimension introduced by multiple antennas. The different layers of data are not necessarily orthogonal to each other and thus weakens the orthogonally between different users in the OFDM-based systems. As an example, consider the uplink channel when Spatial Division Multiple-Access (SDMA) is used. In this scenario, the base station assigns multiple users to share the same time-frequency resource. However, in most scenarios, there is a non-zero correlation between the spatial signatures from these users. As a result, after spatial processing, one user will see part of the energy from other users as interference.
There is an SNR range problem when two wireless terminal users share the same uplink time-frequency resource. This problem is a direct outcome of the channel estimation error, i.e., the channel estimation error of the first user contributes to the interference plus noise term of the second user in the form of |{tilde over (h)}1|2P1, where P1 is the transmit power of user 1 and |{tilde over (h)}1|2 is the estimation error of user 1. Hence, there is a certain correlation between the SNR ranges of these two users such that the SNR difference between these two users should not be too significant for such an approach to work satisfactorily. This is because if a first user transmits at a much larger targeted SNR as compared to a second user, then the received power of the first user is significantly higher than the received power of the second user, and as a result the interference caused by the estimation error, which is proportional to the received power of the first user, will make the decoding of the second user's codeword impossible or nearly impossible. Hence, for interfering users sharing the same uplink frequency-time resource, it is important that they operate at similar SNRs. The sharing of the same uplink time-frequency resource can be, and sometimes is, a partial overlap between segments. A segment is a set of time-frequency resource. Two segments overlap when the two segments have at least some of their time-frequency resource in common. The common time-frequency resource is sometimes called the overlapping portion. The criterion that users sharing the same time-frequency resource operate at similar SNRs is sometimes referred to as the SNR range criterion.
One known approach in wireless communications systems in which a base station communicates with multiple wireless terminals and uses multiple antennas to communicate with a wireless terminal involves the reuse of airlink resources as described in FIG. 1. In this example, each of the segments are of the same size, and the additional segment to be overlapped with the underlying time frequency structure segments is intentionally distributed in equal size portions among a significant number of underlying segments, e.g., four or more underlying segments. Different segments, which are assigned to different wireless terminals, will have different channel vectors. Different segments, which are assigned to different wireless terminals will typically have different dynamic ranges, e.g., different segments can have very different target power levels and/or very different target SNRs.
Drawing 100 is a graph illustrating frequency on the vertical axis 102 and time one the horizontal axis 104. Legend 106 which corresponds to drawing 100 indicates that: (i) segment 1 is identified by descending, from left to right, diagonal line shading as indicated by block 108; (ii) segment 2 is identified by ascending, from left to right, diagonal line shading, as indicated by block 110; (iii) segment 3 is identified by vertical line shading as indicated by block 113; (iv) segment 4 is identified by horizontal line shading as indicated by block 114; (v) segment 5 is identified by dotted shading as indicated by block 116. In drawing 100, it can be observed that segment 5 is contiguous and overlaps with ¼ of each of segments 1, 2, 3, and 4. In this example, segments 1 to 4 do not overlap with each other, while segment 5 overlaps with each of segments 1, 2, 3, and 4.
Drawing 150 is a graph illustrating frequency on the vertical axis 152 and time one the horizontal axis 154. Legend 156 which corresponds to drawing 150 indicates that: (i) segment 1 is identified by descending, from left to right, diagonal line shading as indicated by block 158; (ii) segment 2 is identified by ascending, from left to right, diagonal line shading, as indicated by block 160; (iii) segment 3 is identified by vertical line shading as indicated by block 162; (iv) segment 4 is identified by horizontal line shading as indicated by block 164; (v) segment 5 is identified by dotted shading as indicated by block 166. In drawing 150, it can be observed that segment 5 is non-contiguous and overlaps with ¼ of each of segments 1, 2, 3, and 4.
Consider the examples of FIG. 1 when the segments are being used for uplink signaling from different wireless terminals to a base station with multiple receive antenna elements. The SNR range criterion has to be applied between the user of segment 5 and each of users of segments 1, 2, 3, and 4. Assuming that one of users corresponding to segments 1, 2, 3 and 4 is operating at a low SNR while another one is operating at high SNR, we now have the dilemma that whatever the operating point of the segment 5 user, we will violate the SNR range criteria with one of users 1, 2, 3, and 4. This will lead to an almost always decoding error in one of the segments.
While known techniques allow some use of a resource by multiple devices, there is a need for improved methods and apparatus which allow multiple devices to share the same component resources. It is desirable for the channel vectors of an overlapping resource to be orthogonal from the perspective of reusing the resource in an environment with multiple base station antennas. It would be beneficial for overlapping segments to have the same power level and/or same SNR from the perspective of reusing the resource in an environment with multiple base station antennas. In addition, it would be advantageous if methods and apparatus supported a wide variation of dynamic range for users in the system, e.g., in terms of power levels and/or SNR.